In an electric motor car such as a hybrid vehicle, an electric vehicle and so forth adapted to drive wheels by utilizing driving force of a rotating electrical machine, when a vehicle speed is to be controlled and the vehicle is to be braked, the rotating electrical machine is regeneratively operated to generate braking force and regenerative electric power generated at that time is returned to a battery to charge it therewith, thereby effectively making use of vehicle kinetic energy. Incidentally, in the following, a motor, a generator and a motor generator will be generally referred to as the rotating electrical machine.
That is, although in a conventional vehicle, the kinetic energy of the vehicle has been converted into frictional heat by a friction brake and discarded under braking, in an electric motor car provided with regeneration cooperative brake equipment, driver requested braking force is apportioned to frictional brake force and regenerative brake force, and part of the kinetic energy of the vehicle is recovered as regenerative electric power to charge the battery with it. However, since the part of the braking force is apportioned to the regenerative brake force of the rotating electrical machine, in a case where a battery fail occurs and charging and discharging have been inhibited while the regeneration cooperative brake is being operated, if a regeneration operation of the rotating electrical machine is immediately stopped, the braking force will become insufficient and traveling of the vehicle will become unstable and a feeling of anxiety will be given to the driver.
For example, as control in a case where electric power acceptance to the battery is restricted, a technique that power generation efficiency is lowered by changing the phase of currents so as to lower a power generation amount without changing the torque of the rotating electrical machine is known (for example, Patent Literature 1).